A PCB surface finish is a coating between a component and a bare board PCB. Anyone involved within the printed circuit board (PCB) industry understand that PCB's have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate, making the circuit board unusable. Different surface finishes have their own advantages and disadvantages.
Listed below are some more common surface finishes used in PCB manufacturing.
HASL and lead-free HASL
For decades HASL was one of the most popular surface finish choices. Yet, in recent years, manufacturers have realized its limitations. It slao has its advantages, one of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board.
Excellent Shelf Life
Not good for fine pitch components
Contains Lead (HASL)
OSP (Organic Solderability Preservative)
OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process. OSP isenvironmentally friendly, provides a coplanar surface, is lead-free, and requires low equipment maintenance.
No Way to Measure Thickness
Not good for PTH
Short Shelf life
Immersion Tin (ISn) is a good surface finish choice. Immersion Tin (ISn) is a metallic finish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life, while ISn has more pros than cons.
Top Choice for Press Fit Pin Insertion
Could damage soldermask
Contains Thiourea, a known Carcingen
Exposed Tin on Final Assembly can Corrode
Difficult to Measure Thickness
Not Good for Multiple Reflow/Assembly Processes
Electroless Nickel Immersion Gold (ENIG)
ENIG is quickly becoming the most popular surface finish in the industry. The Nickel is the barrier to the copper and is the surface to which the components are actually soldered to. But ENIG can be expensive, and at times can result in what is commonly known as "black pad syndrome," a buildup of phosphorous between the gold and nickel layers that can result in fractured surfaces and faulty connections.
Long Shelf Life
Good for PTH (Plated Through Holes)
Not good for rework
Damage from ET
Signal Loss (RF)